HRTBs for unspecified lifetimes?

language design
1

Would it ever be possible to write an HRTB for a type whose lifetime variables' names' you don't know? That is, something like for<any lifetimes included in T> fn(T) -> u32?

Motivation

I'm the author of Mockall, a popular mocking library. In order to mock generic methods, Mockall needs to store a completion function as a Box<Any>, and later downcast it to a concrete form. Mockall does not need to downcast the method's generic types; just a Fn object that accepts or returns those types. There is no reason why the completion function can't be 'static; Mockall already handles functions with reference arguments by adding an appropriate HRTB. The problem is that for non-static types more complicated than a reference, Mockall doesn't know what the lifetime arguments are.

One solution would be to allow TypeId for non-'static types. But that dead horse has been beaten enough. My request is simpler. Would it be possible to define an HRTB for every lifetime contained in a given type?

1 Like
2

A way of accomplishing something similar is just T: 'a, which requires T to be valid for (at least) 'a. This, in turn, means any lifetime parameters in T must outlive 'a.

If this isn't what you want, it'd be good to clarify further.

3

Yes, something like that would be good. But the compiler won't accept that, as far as I can tell. Here is the best that I can do: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=1c8a8348d82aac3de274ad89b3fdf3e3 In the function bar, I tried to bound T by a lifetime and put that lifetime in the HRTB. But Rust still thinks that the function isn't 'static. Am I doing something wrong?

4

While definitely not convenient, you can accomplish what I think you want via a GAT indirection to pass a type constructor directly:

trait Bind1 {
    type Bind1<'a>;
}

struct Proxy;
impl Bind1 for Proxy {
    type Bind1<'a> = YourRealType<'a, 'a, 'a>;
}

fn type_id1<Ty: Bind1>() -> TypeId {
    TypeId::of::<for<'a> fn(Ty::Bind1<'a>) -> u32>()
}
5

No, that's no better. Even there the lifetimes of YourRealType are known. If I knew that, I could just write a longer HRTB: for<'a, 'b, 'c>.

6

Yea, it would be great to be able to add constraints involving higher ranked parameters, but currently that's not possible in Rust. I don't remember if this is something that would be tractable with chalk or if it presents other problems.

7

Rustc somewhat supports it internally for &'a &'b T, but doesn't store it in the binder. I believe there is a soundness issue because of that.

8

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